Chlamydiaceae species are well known pathogens, and cause a wide variety of symptoms in both animals and humans (Longbottom and Coulter 2003). Beside the well documented zoonotic potential of Chlamydia (C). abortus and C. psittaci, only recently, a few studies report the isolation of C. suis strains from human eyes (Dean et al. 2013; De Puysseleyr et al. 2014). The source of a zoonotic C. suis infection is the pig, the only animal host for this bacteria. Although C. suis is considered to be the most prevalent chlamydial species, pigs can also become infected by C. pecorum, C. abortus and C. psittaci (Schautteet and Vanrompay 2011). Porcine chlamydial infections are not always associated with symptoms, but if so, they lead to important economic losses due to arthritis, pericarditis, polyserositis, pneumonia, conjunctivitis, enteritis, diarrhea and reproductive failure (Zahn et al. 1995; Andersen and Rogers 1998; Eggemann et al. 2000). Diagnosed infections are commonly treated with tetracycline antibiotics, however, more and more tetracycline resistant C. suis strains are emerging (Andersen and Rogers 1998; Schautteet et al. 2010; Di Francesco et al. 2011; Borel et al. 2012; Schautteet et al. 2013). In vitro transfer of the antibiotic resistance genes between chlamydial species is demonstrated by Suchland et al. (Suchland et al. 2009). These findings raise another point of concern associated with the possible zoonotic character of C. suis. Co-infection of a human individual with a tetracycline resistant (TcR) C. suis and the phylogenetically closely related human pathogen C. trachomatis, creates the ideal setting for transfer of the resistance gene and emergence of a TcR C. trachomatis strain. To avoid creation of multi-resistant strains, comprehensive knowledge about the epidemiology and infection biology of C. suis is required. Recent efforts were done to develop C. suis specific molecular tests (De Puysseleyr et al. 2014; Lis et al. 2014) and to apply these analyses to investigate the presence of C. suis in a human risk population (De Puysseleyr et al. 2014). However, detection of viable C. suis bacteria in animal or human samples provides no information about the presence of an existent infection of this microbe. Serological tests to detect the presence of anti-C. suis antibodies are able to actually prove the presence of an immunological response and thus an infection. Unfortunately, at present, no assay for C. suis specific serodiagnosis is available.
Numerous serological assays targeting chlamydial antigens or anti-chlamydial antibodies are published (Sachse et al. 2009). Assays based on detection of antibodies directed against the surface exposed lipopolysaccharide (LPS) (Wittenbrink et al. 1991) or the synthetic neoglycoconjugate containing the trisaccharide alphaKdo(2→4)alphaKdo(2→4)alphaKdo (Kdo, 3-deoxy-D-manno-oct-2-ulopyranosonic acid) which represents a structure of the lipopolysaccharide (LPS) (Brade et al., 2000) and against the full length proteins or peptides from the major outer membrane protein (MOMP) (Vanrompay et al. 2004; Medac, Wedel, Germany), polymorphic outer membrane proteins (Pmps) (Longbottom et al. 2001; Longbottom et al. 2002) or whole elementary body (EB) preparations (Di Francesco et al. 2006), were developed with varying degrees of success in terms of sensitivity and specificity. However, in some cases, cross-reactivity between Chlamydia species can hinder interpretation of results (Donati et al. 2009). Hoelzle et al. demonstrated the suitability of the full length recombinant MOMP proteins of C. suis, C. abortus and C. pecorum in ELISA assays to identify the infecting chlamydial species (Hoelzle et al. 2004). However, until now, this approach has not been further verified in animals in the field. Furthermore, a recombinant protein fragment of the Pmp90 was successfully used for serological diagnosis of C. abortus infections (Longbottom et al. 2001). The Pmps are considered as important virulence factors and several studies demonstrated the induction of an immune response (Grimwood and Stephens 1999; Niessner et al. 2003; Wehrl et al. 2004) and even protective immunity (Cevenini et al. 1991). Tan et al. (2009) demonstrated that the Pmps elicit various serologic responses in C. trachomatis-infected patients. However, differences in the strengths and specificities of the Pmp subtype-specific antibody reactivity related to gender and clinical outcome were observed. This indicates that the Pmp gene family forms the basis of a mechanism of antigenic variation. The PmpD protein of C. abortus was recognized as a major antigen using sera of experimentally infected ewes. In addition, also the MOMP protein and the translocated actin recruitment protein (Tarp) were identified as reactive proteins in this study (Marques et al. 2010; Forsbach-Birk et al. 2013). The Tarp protein is also predominantly recognized by antibodies from humans infected with C. trachomatis (Wang et al. 2009).
Specific detection of anti-C. suis antibodies can provide the evidence for an existent infection in pigs and humans, an essential factor in the study of the zoonotic transfer of this microbe. Unfortunately, to date, no sensitive C. suis specific assay is available for animal or human serodiagnosis and no studies to identify the major reactive antigens of C. suis have been published until today. In fact, seroprevalence studies in pigs are based on detection of antibodies against LPS, MOMP and whole EB preparations and serological cross-reactions with antibodies against other chlamydial species or other pathogens do occur (Schautteet and Vanrompay 2011).
There is thus currently still a need for antigenic peptides that are useful in the detection or diagnosis of Chlamydia suis in a subject.